Coagulation sedimentation is a commonly used unitary operation in drinking water and waste-water treatment technique. Aluminum-based and iron-based inorganic coagulants are the most commonly used coagulants nowadays. However, the water treated with aluminum-based coagulants still contains a large amount of residue aluminum which exceeds the specified standard. Long-term drinking of such water will cause dementia. Iron-based coagulants, although without biological toxicity, may lead to coloration of the effluent is excessively used. In addition, the iron ions generated during the hydrolysis cause strong corrosion to the water treatment equipment. In addition, when aluminum-based or iron-based coagulant is used, a large amount of chemical sludge is produced, and improper sludge treatment may lead to serious secondary pollution. Therefore, searching for new and highly efficient wastewater treatment inorganic coagulants which satisfy the requirements for water and the waste water treatment has become a hot research subject in recent years.
Titanium salt is a new water treatment agent which has gradually attracted the attention of researchers in recent years. Compared with the conventional iron and aluminum coagulants, titanium salt has a strong electrical neutralization capability, the formed floc is large and compact and the sedimentation speed is higher, which is more favorable to solid-liquid separation and improvement of the decontamination effect. In addition, the titanium salt causes less corrosion to the water treatment equipment, and would not cause the problem of chrominance. Further, the titanium has no toxicity, and thus the titanium resident in the water would not cause damages to the human health. At the same time, it has been reported in a study that TiCl4, in cold conditions, achieves a better coagulation effect that the traditional aluminum or iron salt coagulant does, which is highly significance for the practice of treatment of waste water in China's northwest and northeast in winter. In addition, the related studies report that when titanium sulfate and titanium tetrachloride are used as coagulants, the removal rate of the organic matters is obviously higher than that achieved when the traditional aluminum or iron salt is used. After the resulting sludge upon coagulation is subjected to high temperature calcination, titanium dioxide photocatalyst which has a great application value can be obtained, which effectively solves the problem of a large amount of sludge is caused during the coagulation process, and achieves double effects of water purification and sludge reuse.
However, during use of the titanium tetrachloride, titanium sulfate and other titanium salt coagulants, because of their strongly acidity and Ti4+ thereof at hydrolysis will release a lot of H+, pH of the water resulted from the coagulation is relatively low, which affects further use of the water. Further, titanium tetrachloride, titanium sulfate and other titanium salts have a very fast hydrolysis rate in water, and a titanium hydroxyl hydrolysis product which is the most effective cannot be formed, thereby affecting the coagulation effect. By modifying the coagulant in an artificial manner, a better coagulation effect may be achieved. Based on the hydrolysis characteristics of the titanium salt, at present the titanate inorganic coagulant is optimized mainly by using the method for preparing a polymerization titanate coagulant or an organic polymer floc formulation. Chinese patent application No. CN102701387A has disclosed a titanium tetrachloride-poly dimethyl diallyl ammonium chloride inorganic-organic compound flocculant and a preparation method thereof. According to the invention, the deficiency of the single flocculant is addressed by combined use of an inorganic flocculant and an organic polymer flocculant, the application range thereof is enlarged, the residual metal ion concentration is lowered, and secondary pollution is relieved. Chinese patent application No. CN103964554A has disclosed a polymeric titanium sulfate-poly dimethyl diallyl ammonium chloride coagulant complex. According to the invention, by using the hydrolysis characteristics of the titanium sulfate, a polymerization titanium sulfate is prepared first by adding a base to pre-hydrolyze the titanium sulfate, and then a coagulant having a high stability is obtained by complexing with the organic polymer flocculant. Although the methods of pre-preparing the polymerization titanium sulfate and complexing with the organic polymer coagulant improve the coagulation effect and enlarge the application scope of the coagulant, these two materials both need to be on-site prepared before use, the preparation is complicated, and the prepared liquid cannot be stored for a long time, which reduces the time effectiveness of the titanate coagulant during the water treatment. Therefore, a more highly efficient and more convenient titanate coagulant preprocessing method has a great significant.
The sol-gel method is a commonly used method for preparing the titanium dioxide materials, wherein the inorganic salt or titanium ester is used as the raw material, a sol is directly formed by hydrolysis, and a gel is further obtained via polycondensation. Titanium dioxide particles are obtained after the gel experiences calcination. During the sol-gel preparation, generally a hydrolysis inhibitor is added to regulate the hydrolysis-polycondensation rate to control synthesis of the gel. The properties of the finally resulted material may be implemented by adjusting a proportion of the components at the sol stage. The process of preparing the titanium dioxide by means of sol-gel method generally produces the final products TiO2 nanoparticles, which are used in the field such as photocatalysis and hydrolysis. However, the intermediate TiO2 dry gel in the synthesis process is less concerned. At present, it is only reported that the TiO2 dry gel is used as an adsorbent. For example, Chinese patent No. ZL201210509803.3 has disclosed a TiO2 dry gel material which can be used as an adsorbent, and a preparation method and use thereof. In this patent, tetrabutyl titanate is used as a titanium source, acetylacetone is used as a hydrolysis inhibitor, by adjusting the proportion of raw materials, a TiO2 dry gel is formed by means of the sol-gel synthesis. The dry gel has good adsorption properties against azo dyes, the adsorption capacity thereof is comparable to that of the activated carbon material. It should be noted that the TiO2 dry gel is insoluble in water. For a coagulant, good water solubility is an important prerequisite for bringing its flocculation and sedimentation properties thereof to full play. At present, there is no yet disclosure of the water-soluble TiO2 dry gel. The most prominent problem during the use of an inorganic titanium salt as the coagulant is that the hydrolysis rate is high and thus the pH of the resulted water is low. To overcome this problem, by the sol-gel method, inorganic titanium salt is modified, and the resulted water-soluble TiO2 dry gel has a controllable hydrolysis rate, and may be potentially used as a coagulant. There is still no report on using the TiO2-based material as a coagulant for water treatment.